Mobilization of Nitrogen in Fruiting Plants of a Cultivar of Cowpea

Abstract

Patterns of flow of nitrogen were constructed for the post-anthesis development of symbiotically-dependent cowpea (Vigna unguiculata Walp. cv. Vita 3-Rhizobium CB756). Nitrogen fixed after flowering contributed 40% of the fruits' total intake of N, mobilization of N fixed before flowering the remaining 60%. Leaflets, nodulated root, stem plus petioles, and peduncles contributed mobilized N in the approximate proportions 5: 2: 1: 1 respectively. Each fruit drew on all available current sources of N, but N from leaves was distributed preferentially to closest fruit(s), and lower fruits monopolized the N exported from nodulated roots during late fruiting. Rates of nitrogen fixation declined parallel with decreasing net photosynthesis of shoots. Leaflets at upper reproductive nodes mobilized 70–77% of their N and declined steeply in net photosynthesis rate per unit chlorophyll or per unit ribulose-l, 5-bisphosphate carboxylase (RuBPCase)2 before abscising during mid- to late fruiting, whereas leaflets at lower vegetative nodes (1–3) mostly failed to abscise, lost 44–57% of their N and maintained photosynthetic activity throughout fruiting. Peptide hydrolase activity was examined in extracts of leaflets, roots and nodules, by autodigestion of extracts, or in assays using bovine haemoglobin and purified RuBPCase isolated from cowpea as substrates. Hydrolase activities during fruiting were broadly related to N loss from plant organs, but asynchrony in peaks of activity against different protein substrates indicated distinct groups of hydrolases within single organs. Hydrolase activity of leaflet extracts against RuBPCase was highly and positively correlated with in vivo rates of loss of RuBPCase from the same leaflets, and preferential degradation of this protein occurred during leaflet senescence.